Although it isn't clear whether W'Kabi and Nakia were
"dusted" or not by the snap,
is there any information regarding their whereabouts when Proxima Midnight and Cull Obsidian led the Outriders' attack on Wakanda?
Although the information on W'Kabi isn't revealed just yet Nate Moore has gone on record saying Marvel does know where they are and we do know where Nakia was.
We definitely know where Nakia is and we know where W'Kabi is, and that was stuff we talked about with the Russo brothers and with Markus and McFeely. So I think as you move forward through the Marvel Cinematic Universe those stories will be told, if not right away pretty soon.
He has also said it will likely be revealed in Avengers: Endgame so that is probably why they haven't commented on it yet. Having watched the film now though I can say that this wasn't the case and Endgame did not give us any further details. However, in Phase 4 I believe there is going to be a Black Panther sequel so we should find out there.
I don't want to spoil it, but Avengers 4 obviously will deal with the fallout of Avengers 3 and I think you may get some hints there.
Joe Russo has commented that Nakia was on missions "deep undercover" during Avengers: Infinity War.
When asked why she didn’t appear in “Infinity War,” Joe said, “She’s on missions.” Anthony added, “Yeah, deep undercover.”
It is highly possible that she was at the first Wakandan International Research Center or running missions for it as she was appointed to oversee the social outreach by T'Challa.
T'Challa: And that building, and that one over ther.... This will be the first Wakandan International Research Center.
Shuri smiles at the surprise.
T'Challa: Nakia will oversee the social outreach... And you will spearhead the science and information exchange.
W'Kabi was likely in prison considering he had "betrayed" T'Challa and therefore Wakanda and had surrendered to Okoye which ended the battle.